Indianapolis, Indiana, May 5, 2025 - Copper Mountain Technologies' (CMT) Brian
Walker, Sr. RF Engineer, SME, has published a new white paper entitled "Near and
Far-Field Antenna Measurements: A Concise Overview." He asserts that accurate
antenna testing is essential for validating key design parameters like gain,
beamwidth, radiation pattern, polarization, and sidelobe levels. Measurements
can be conducted in either the far-field or near-field regions, each with
distinct advantages and challenges. Far-field testing, ideal for well-defined
angular patterns, includes methods such as outdoor ranges, compact ranges, and
anechoic chambers, though it requires significant space and environmental
control. Near-field testing, which involves scanning a probe around the antenna
and transforming data numerically, overcomes space constraints and provides full
3D pattern data but demands precise positioning and complex processing. Both
approaches must mitigate errors like multipath interference, probe misalignment,
and truncation effects. The choice between near- and far-field methods depends
on antenna size, frequency, accuracy requirements, and available resources.
Copper Mountain Technologies offers specialized solutions, including compact
mmWave test systems, to facilitate efficient and precise antenna
characterization.
Near and Far-Field Antenna Measurements: A Concise
Overview
Brian Walker, Sr. RF Engineer, SME
Intro:
Near and
Far-Field Antenna Measurement Guide
Figure 1 – Dipole Near and Far Fields.
Accurate antenna testing is crucial for validating design parameters such as
antenna gain measurement, beamwidth, radiation pattern, polarization, and sidelobe
levels. These characteristics can be measured in either the far-field or near-field
regions of an antenna. Each approach has distinct advantages, limitations, and requirements.
This paper provides a concise yet comprehensive summary of the underlying theory,
measurement methodologies, and practical considerations involved in near- and far-field
antenna testing.
Antenna Measurement Goals
Antenna measurements aim to quantify the radiation characteristics in free space.
The desired output typically includes:
- Radiation patterns (amplitude and phase vs. angle)
- Gain and directivity
- Beamwidth and sidelobe levels
- Cross-polarization discrimination
- Input impedance and VSWR
Measurements must be made in environments free of reflections, obstructions,
or near-field effects that would distort the intrinsic behavior of the antenna under
test (AUT).
Near vs. Far-Field Antenna Measurements
The electromagnetic field surrounding an antenna is generally divided into three
regions:
- Reactive Near Field: Very close to the antenna, where reactive fields dominate
and energy storage outweighs radiation.
- Radiating Near Field (Fresnel Region): Transition zone where radiation begins
but field patterns still vary with distance.
- Far Field (Fraunhofer Region): The region where the radiated fields form well-defined
angular patterns independent of distance, ideal for antenna characterization.
The boundary between the near-field and far-field regions is typically given
by:
R>(2D2)/λ
where:
R is the distance from the antenna, 𝐷 is the largest dimension of the antenna
aperture, λ is the wavelength.
This rule ensures sufficient distance for planar wavefront formation.
Far-Field Antenna Measurement Types
Outdoor Ranges (Ground and Elevated): These use large open areas to isolate the
AUT and source antenna. Elevated ranges reduce ground reflections, while slant ranges
position antennas above a reflective surface with absorbers to control multipath.
Figure 2 - mmWave OTA Antenna Test System.
Compact Ranges:
Use large reflectors to collimate spherical waves into planar wavefronts. Useful
when long physical distances are impractical. Accuracy depends on reflector design,
surface quality, and feed illumination.
For mmWave measurement, the far-field may begin at a distance of only 77 cm.
For this case, the measurement could be conveniently performed with a
small anechoic
chamber, such as one offered by Copper Mountain Technologies in partnership
with MilliBox. The Copper Mountain Technologies anechoic chamber in Figure 2 may
be used for characterizing antennas in the 18 to 95 GHz range.
Figure 3 - Anechoic Chamber.
Figure 4 - Outdoor Measurement Range.
Figure 5 - Yagi Antenna Pattern.
RF Anechoic Chambers:
Indoor facilities are lined with RF-absorbing material to simulate free-space
conditions. This allows precise environmental control and repeatable measurements
but is limited by size and cost, especially at low frequencies where required dimensions
grow large.
Outdoor Testing
With an outdoor range, the measurement is performed far enough away from the
antenna to guarantee a planar, far-field measurement. The largest source of error
is due to ground reflection. The source antenna may be on the top of a slope to
minimize this multipath error.
Key Parameters for Antenna Measurement
- Antenna Pattern (Figure 5): Measured by rotating the AUT or probe and recording
amplitude/phase as a function of angle.
- Polarization Discrimination: Cross-polarization levels reveal misalignment or
undesired modes.
- Gain Measurement: Requires comparison to a reference antenna with known gain,
often using substitution or comparison methods. The three-antenna method may be
used if none of the antenna gains are known.
Near-Field Antenna Measurement Techniques
Near-field testing overcomes space constraints and provides full 3D pattern data,
especially useful for electrically large or high-gain antennas. It involves scanning
a probe across a plane, cylinder, or sphere surrounding the AUT and numerically
transforming the data to the far field using Fourier-based methods.
Because Maxwell's equations are linear, if the fields can be mapped on a closed
surface surrounding the AUT, fields anywhere else may be predicted accurately. A
near-field probe is moved in a grid around the AUT or in a square in front of a
highly directional antenna, and the E or H field is measured.
Antenna Scanning Geometries
Planar Scanning: The AUT is stationary while a probe moves over
a 2D grid. Applicable to antennas with narrow beamwidths and planar apertures. Requires
truncation correction.
Cylindrical Scanning: The probe moves along the length of a
cylinder surrounding the AUT, suitable for antennas with elongated geometries (e.g.,
linear arrays). Allows 360° coverage.
Spherical Scanning: Captures data over a full sphere, ideal
for arbitrary radiation patterns, including low sidelobes and wide-angle coverage.
Most general but mechanically complex and time consuming.
Probe Correction and Polarization
The probe used in near-field scanning is not ideal; its own radiation pattern
must be de-embedded from the measurement. Accurate far-field transformation relies
on known probe characteristics and compensation algorithms.
Polarization mismatch between the probe and AUT must be minimized. Measurements
often include multiple polarizations (e.g., linear, circular) to fully characterize
the radiation behavior.
Mathematical Transformations
The key to near-field testing is the ability to compute far-field parameters
from measured near-field data using transformation algorithms:
- Fourier Transforms: Planar scans use 2D FFTs.
- Spherical Harmonic Expansions: Spherical scans leverage modal decomposition.
- Mode Filtering: Removes spurious or noise-induced artifacts.
Field quantities are processed as complex vectors, and transformations correct
for probe effects, scan geometry, and measurement truncation.
Advantages of Near-Field Testing
- Requires less physical space than far-field methods.
- Supports automated, precise, and rapid scanning.
- Enables high accuracy and full 3D radiation characterization.
- Indoor environment allows temperature, humidity, and EM noise control.
However, it requires:
- High precision mechanical positioning systems.
- Accurate probe characterization.
- Complex data processing and transformation software.
Measurement Errors and Uncertainties
Sources of Error in Antenna Measurement
- Multipath and Reflections: Can distort both amplitude and phase.
- Probe Positioning Error: Causes phase errors in transformation.
- Cable Motion and Flexing: Induces amplitude/phase variation.
- Truncation and Sampling Error: Improper scan size or resolution distorts the
far-field transform.
- Temperature Drift: Affects electronics and mechanical stability.
Mitigating Errors in Antenna Measurement
- Use of time-gating or spatial filtering to isolate direct signal paths.
- Calibration routines with known reference sources.
- Meticulous probe alignment and motion system calibration.
- Environmental control within chambers.
Quantitative uncertainty budgets can be developed following standardized procedures
such as those from IEEE or ISO to document overall accuracy.
In Conclusion: Measuring
Near
and Far-Field Communication Antennas
Far-field and near-field antenna measurement techniques each offer unique strengths.
Far-field testing is conceptually simple and widely used, but space and environmental
constraints can be limiting. Near-field testing allows detailed and compact measurement
setups with complete pattern recovery but requires sophisticated processing and
error correction.
Ultimately, the choice depends on antenna size, frequency, measurement goals,
available space, cost, and desired accuracy. Copper Mountain Technologies (CMT)
offers world-class Vector
Network Analyzers and compact
Over
the Air (OTA) mmWave test systems for an economical solution to antenna characterization.
Advanced features like time gating are a standard feature of nearly all CMT's VNAs.
